David Letterman has championed all kinds of music during his long career on late-night TV. But no genre has benefited more from the talk-show host than Americana

When David Letterman retires this month from CBS’s “Late Show,” musicians everywhere will be singing the blues as the talk-show host winds up more than three decades as their champion. Mr. Letterman’s departure is a particular loss for a genre enjoying a renaissance: Americana.

“Dave was celebrating Americana artists before we knew what we were called,” says Emmylou Harris, one of the scene’s matriarchs, who first performed on Mr....